High-molecular aliphatic polyester and process for producing the same

ABSTRACT

Disclosed herein are a high-molecular weight aliphatic polyester, whose molecular weight has been highly increased by a chain-lengthening reaction of a ring-opening (co)polymer of at least one cyclic ester selected from the group consisting of glycolide and lactide with an oxazoline compound, and a production process thereof. The molecular weight of the high-molecular weight aliphatic polyester is highly increased to the extent that a rate of increase in molecular weight represented by a ratio (Mw 2 /Mw 1 ) of a weight average molecular weight (Mw 2 ) of a ring-opening (co)polymer after the chain lengthening to a weight average molecular weight (Mw 1 ) of the ring-opening (co)polymer before the chain lengthening amounts to at least 1.10.

TECHNICAL FIELD

The present invention relates to a high-molecular weight aliphaticpolyester, whose molecular weight has been highly increased by areaction of a ring-opening (co)polymer of at least one cyclic esterselected from the group consisting of glycolide and lactide with achain-lengthening agent, and a production process thereof. Thehigh-molecular weight aliphatic polyester according to the presentinvention is high in molecular weight and excellent in heat resistanceand can be used in a wide variety of fields as extruded products such assheets, films and fibers, compression-molded products, injection-moldedproducts, blow-molded products, composite materials (multi-layer filmsand multi-layer containers), and other formed or molded products.

BACKGROUND ART

Aliphatic polyesters such as polyglycolic acid and polylactic acid arebiodegradable resins degraded by microorganisms or enzymes present inthe natural world such as soil and sea because they contain aliphaticester linkages in their molecular chains. These aliphatic polyesters areuseful as medical polymer materials for surgical sutures, artificialskins, etc. because they have degradability and absorbability in vivoand biocompatibility (for example, U.S. Pat. No. 3,297,033).

Among the aliphatic polyesters, the polyglycolic acid is markedlyexcellent in gas barrier properties, and so its new uses have beendeveloped as sheets, films, containers, etc. (for example, JP-A10-60136, JP-A 10-80990, JP-A 10-138371 and JP-A 10-337772).

The polyglycolic acid can be produced by dehydration polycondensation ofglycolic acid, dealcoholization polycondensation of an alkyl glycolate,desalting polycondensation of a glycolic acid salt, or the like.However, these polycondensation reactions are difficult to provide ahigh-molecular weight polyglycolic acid. To the contrary, whenglycolide, which is a bimolecular cyclic ester (also referred to as“cyclic dimer”) of glycolic acid, is subjected to ring-openingpolymerization, comparatively high-molecular weight polyglycolic acidcan be provided. The ring-opening polymer of the glycolide may be calledpolyglycolide in some cases.

A polycondensation reaction of lactic acid, lactate or lactic acid saltis also difficult to provide polylactic acid as a high-molecular weightpolymer. Therefore, the polylactic acid is generally synthesized byring-opening that is a bimolecular cyclic ester of lactic acid. Thering-opening polymer of the lactide may be called polylactide in somecases. The glycolide and lactide may also be subjected to ring-openingcopolymerization.

With the advancement of technical development as to ring-opening(co)polymers of cyclic esters and the attempt to develop their new uses,the ring-opening (co)polymers are required to improve their mechanicalstrength, heat resistance, forming or molding and processing ability,and the like. In particular, since physical properties of thering-opening (co)polymers, such as mechanical strength, mainly depend ontheir molecular weights, there is a strong demand for increase of theirmolecular weights.

According to the ring-opening (co)polymerization of the cyclic ester ofthe glycolide or lactide, a comparatively high-molecular weightaliphatic polyester can be provided compared with the polycondensationof glycolic acid, lactic acid or the like. However, it has not been yetsufficient in the light of the state of requirements in recent years,and problems to be solved have been left to the increase in molecularweight.

First, in order to synthesize a high-molecular weight aliphaticpolyester by ring-opening (co)polymerization of a cyclic ester, it isnecessary to use a high-purity monomer(s). However, the glycolide orlactide is difficult to highly purify it, in addition to the fact thatits own for a purifying treatment. It has therefore been extremelydifficult to supply a high-molecular weight aliphatic polyester in anindustrially great amount at a low price according to the productionprocess in which the high-purity monomer(s) must be used.

Second, the aliphatic polyester tends to greatly vary its molecularweight according to slight changes in polymerization conditions such aspolymerization temperature, polymerization time, polymerizationpressure, and the kinds and amounts of a catalyst and additives inaddition to the purity of the monomer(s). It has therefore beendifficult to stably produce a high-molecular weight aliphatic polyester.

Third, even when a high-molecular weight aliphatic polyester issynthesized under strict control of the purity of the monomer(s) and thepolymerization conditions, the level of its molecular weight is notalways said to be sufficient. For example, the weight average molecularweight (Mw) of polyglycolic acid obtained by the ring-openingpolymerization of glycolide is about 100,000. In order to produce aformed or molded product having high physical properties, it isnecessary to further increase the molecular weight of the aliphaticpolyester.

Since the physical properties of an aliphatic polyester, such asmechanical strength, mainly depend on its molecular weight as describedabove, there is a demand for development of a process for increasing themolecular weight of the aliphatic polyester by a simple and cheapmethod. The conventional aliphatic polyesters have involved a problemthat heat resistance is insufficient, and so they tend to undergothermal decomposition when exposed to high-temperature conditions upontheir melt processing or the like. In addition, a molecular weightdistribution is desirably relatively broad from the viewpoint of formingor molding and processing ability. It has however been difficult toprovide an aliphatic polyester having a high molecular weight and abroad molecular weight distribution by the conventional productionprocesses.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a high-molecularweight aliphatic polyester that is a ring-opening (co)polymer of acyclic ester such as glycolide or lactide, whose molecular weight hasbeen highly increased, and whose heat resistance and forming or moldingand processing ability have been improved.

Another object of the present invention is to provide a process forproducing a high-molecular weight aliphatic polyester, by which themolecular weight of the resulting polymer can be easily increased to adesired molecular weight without need of always using high-purityglycolide or lactide as a starting material, and heat resistance andforming or molding and processing ability are also improved.

The present inventors have carried out an extensive investigation with aview toward achieving the above objects. As a result, it has been foundthat a ring-opening (co)polymer of at least one cyclic ester selectedfrom the group consisting of glycolide and lactide is subjected to achain-lengthening reaction with an oxazoline compound, whereby the chainof the ring-opening (co)polymer is lengthened to highly increase itsmolecular weight. Reaction conditions for the chain-lengtheningreaction, such as the amount of the oxazoline compound used, reactiontemperature, and reaction time are controlled, whereby the molecularweight and molecular weight distribution of the resulting polymer can becontrolled, and an aliphatic polyester, whose molecular weight has beenhighly increased to the extent that the conventional processes have beenunable to achieve, can be provided.

According to the process of the present invention, even the simple useof the oxazoline compound as the chain-lengthening agent can produce ahigh-molecular weight aliphatic polyester. In addition, thehigh-molecular weight aliphatic polyester obtained by the productionprocess according to the present invention becomes high in weightloss-starting temperature on heating and is hence markedly improved inheat resistance. Since the high-molecular weight aliphatic polyesteraccording to the present invention is moderately broad in molecularweight distribution, the forming or molding and processing abilitythereof is improved. In the present invention, the oxazoline compoundacts as a chain-lengthening agent, not assume an action as a chainterminator. The present invention has been led to completion on thebasis of these findings.

According to the present invention, there is thus provided ahigh-molecular weight aliphatic polyester, whose molecular weight hasbeen highly increased by a chain-lengthening reaction of a ring-opening(co)polymer of at least one cyclic ester selected from the groupconsisting of glycolide and lactide with an oxazoline compound to theextent that a rate of increase in molecular weight represented by aratio (Mw₂/Mw₁) of a weight average molecular weight (Mw₂) of aring-opening (co)polymer after the chain lengthening to a weight averagemolecular weight (Mw₁) of the ring-opening (co)polymer before the chainlengthening amounts to at least 1.10.

According to the present invention, there is also provided a process forproducing a high-molecular weight aliphatic polyester, which comprisessubjecting a ring-opening (co)polymer of at least one cyclic esterselected from the group consisting of glycolide and lactide to achain-lengthening reaction with an oxazoline compound to highly increasethe molecular weight thereof to the extent that a rate of increase inmolecular weight represented by a ratio (Mw₂/Mw₁) of a weight averagemolecular weight (Mw₂) of a ring-opening (co)polymer after the chainlengthening to a weight average molecular weight (Mw₁) of thering-opening (co)polymer before the chain lengthening amounts to atleast 1.10.

BEST MODE FOR CARRYING OUT THE INVENTION

1. Ring-Opening (Co)Polymer

The ring-opening (co)polymer of a cyclic ester can be obtained bysubjecting glycolide, lactide or a mixture of glycolide and lactide toring-opening (co)polymerization. The glycolide is a bimolecular cyclicester of glycolic acid and can be suitably produced by, for example,depolymerization of a glycolic acid oligomer. The lactide is abimolecular cyclic ester of lactic acid and may be any of an L body, a Dbody, a racemic body and a mixture thereof.

Among these, the glycolide is suitable for use as a starting materialbecause it is difficult to purchase a high-purity product in a greatamount and at a low price. The reason for it is that according to theprocess of the present invention, a high-molecular weight polyglycolicacid (polyglycolide) can be finally obtained without need of alwaysusing high-purity glycolide.

Since the polyglycolic acid is excellent in gas barrier properties, amonomer comprising glycolide as a main component is desirably used whenthe resulting polyglycolic acid is used in application fields of sheets,films, containers, composite materials, etc. In the monomer comprisingglycolide as a main component, the proportion of the glycolide ispreferably at least 55% by weight, more preferably at least 70% byweight, particularly preferably at least 90% by weight. It goes withoutsaying that the glycolide may be used by itself.

In the present invention, glycolide, lactide or a mixture thereof isused as a monomer, and any of cyclic monomers such as lactones (forexample, β-propiolactone, β-butyrolactone, pivalolactone,γ-butyrolactone, δ-valero-lactone, β-methyl-δ-valerolactone,ε-caprolactone, etc.), trimethylene carbonate and 1,3-dioxane may beused in combination as another comonomer. These comonomers are used in aproportion of generally at most 45% by weight, preferably at most 30% byweight, more preferably at most 10% by weight. If the proportion ofthese comonomers is too high, the crystallinity of a ring-openingcopolymer formed when used in combination of, for example, glycolide isimpaired, and its heat resistance, gas barrier properties, mechanicalstrength, etc. are deteriorated.

The ring-opening (co)polymerization of the cyclic ester is preferablyconducted in the presence of a small amount of a catalyst. No particularlimitation is imposed on the catalyst. As examples thereof, may bementioned tin compounds such as tin halides (for example, tindichloride, tin tetrachloride, etc.) and organic tin carboxylates (forexample, tin octanoates such as tin 2-ethylhexanoate); titaniumcompounds such as alkoxytitanates; aluminum compounds such asalkoxyaluminum; zirconium compounds such as zirconium acetylacetone; andantimony compounds such as antimony halides and antimony oxide. Theamount of the catalyst used is preferably about 1 to 1,000 ppm, morepreferably about 3 to 300 ppm in terms of a weight ratio based on thecyclic ester.

The ring-opening (co)polymerization of the cyclic ester may be conductedby either bulk polymerization or solution polymerization and isoptional. In many cases, however, the bulk polymerization is adopted. Ahigher alcohol such as lauryl alcohol, water or the like may be used asa molecular weight modifier for the purpose of regulating the molecularweight of the resulting polymer. In addition, a polyhydric alcohol suchas glycerol may be added for the purpose of improving the physicalproperties of the resulting polymer.

A polymerizer for the bulk polymerization may be suitably selected fromamong various kinds of apparatus such as extruder type, vertical typehaving a paddle blade, vertical type having a helical ribbon blade,horizontal type such as an extruder type or kneader type, ampoule type,plate type and annular type. Various kinds of reaction vessels may beused for the solution polymerization.

The polymerization temperature can be suitably preset within a range offrom 120° C. which is a substantial polymerization-initiatingtemperature, to 300° C. as necessary for the end application intended.The polymerization temperature is preferably 130 to 250° C., morepreferably 140 to 230° C., particularly preferably 150 to 225° C. If thepolymerization temperature is too high, a polymer formed tends toundergo thermal decomposition. The polymerization time is within a rangeof from 3 minutes to 20 hours, preferably from 5 minutes to 18 hours. Ifthe polymerization time is too short, it is hard to sufficiently advancethe polymerization. If the polymerization time is too long, a polymerformed tends to be colored.

No particular limitation is imposed on the molecular weight of thering-opening (co)polymer of the cyclic ester. Even in a ring-opening(co)polymer having a relatively low molecular weight, its molecularweight can be highly increased by subjecting it to a chain-lengtheningreaction with an oxazoline compound. In order to efficiently increasethe molecular weight by the reaction with the oxazoline compound toprovide an aliphatic polyester having a sufficiently high molecularweight, the weight average molecular weight (Mw) of the ring-opening(co)polymer is of the order of at least 30,000, preferably 30,000 to500,000, more preferably 30,000 to 110,000.

2. Oxazoline Compound

Examples of the oxazoline compound used in the present invention include2-oxazoline compounds such as 2-oxazoline, 2-methyl-2-oxazoline,2-isopropyl-2-oxazoline, 2-butyl-2-oxazoline and 2-phenyl-2-oxazoline;2,2′-bis(2-oxazoline) compounds such as 2,2′-bis(2-oxazoline),2,2′-methylene-bis(2-oxazoline), 2,2′-ethylene-bis(2-oxazoline),2,2′-trimethylene-bis(2-oxazoline),2,2′-tetramethylene-bis(2-oxazoline),2,2′-hexamethylene-bis(2-oxazoline),2,2′-octamethylene-bis(2-oxazoline),2,2′-ethylene-bis-(4,4′-dimethyl-2-oxazoline),2,2′-p-phenylene-bis(2-oxazoline) and 2,2′-m-phenylene-bis(2-oxazoline);and bis-(2-oxazolinylcyclohexane) sulfide and polymeric compounds withat least 2 oxazoline ring structures introduced at molecular chainterminals or into side chains thereof.

The oxazoline compound is preferably a compound having at least 2oxazoline ring structures in its molecule from the viewpoint ofefficiently performing the chain-lengthening reaction.

Among the oxazoline compounds, are preferred compounds having 2oxazoline ring structures in their molecules and represented by thefollowing formula (1):

In the formula, A is a single bond or a divalent organic group. As thedivalent organic group, is preferred —(CH₂)_(n)—(n is an integer of 1 orgreater, preferably 1 to 20 or a phenylene group. R¹ and R² are,individually of each other, an alkyl group (having 1 to 10 carbon atoms)cycloalkyl group, phenyl group or the like, with an alkyl group having 1to 5 carbon atoms being preferred.

Among the compounds having 2 oxazoline ring structures in theirmolecules, 2,2′-m-phenylene-bis(2-oxazoline) represented by thefollowing formula (2):

is particularly preferred because it is easily available and excellentin reactivity.

The amount of the oxazoline compound used is preferably 0.001 to 10parts by weight, more preferably 0.05 to 7 parts by weight, particularlypreferably 0.1 to 5 parts by weight per 100 parts by weight of thering-opening (co)polymer of the cyclic ester. If the amount of theoxazoline compound used is too little, it is difficult to sufficientlyincrease the molecular weight of the ring-opening (co)polymer. If theamount is too great, the chain-lengthening effect shows a tendency tosaturate, and so such a too great amount is not economical. Ahigh-molecular weight aliphatic polyester having a desired molecularweight can be obtained by controlling the amount of the oxazolinecompound used.

3. Production Process of High-Molecular Weight Aliphatic Polyester

The oxazoline compound may be added to the reaction system during aring-opening (co)polymerization reaction of the cyclic ester or afterthe reaction. In order to stably obtain a high-molecular weightaliphatic polyester having a desired molecular weight, however, theoxazoline compound is desirably added to the resultant ring-opening(co)polymer after completion of the polymerization reaction. Theoxazoline compound may be added at a time or in 2 or more portions.

The temperature of a reaction of the ring-opening (co)polymer with theoxazoline compound is within a range of preferably 100 to 300° C., morepreferably 150 to 280° C. It is particularly preferred that thisreaction temperature be not lower than the melting temperature of thering-opening (co)polymer, but not higher than 300° C., more preferablynot lower than the melting temperature, but not higher than 280° C. Thetime of the reaction varies according to the reaction temperature and isof the order of preferably from 30 seconds to 100 minutes, morepreferably 1 to 60 minutes, still more preferably 5 to 40 minutes,particularly preferably 10 to 30 minutes.

Although the details of the reaction mechanism of the ring-opening(co)polymer with the oxazoline compound are not clearly known at thepresent stage, the present inventors consider to be as follows. Anoxazoline compound such as 2-oxazoline is known to exhibit behavior ofliving polymerization if selecting conditions. On the other hand, aring-opening (co polymer of glycolide or lactide has a carboxyl group onat least one terminal. A linkage (O—C) between a carbon atom at a5-position of an oxazoline ring and an oxygen atom is severed byinteraction between this carboxyl group and the oxazoline ring to openthe oxazoline ring, and an oxygen atom of the carboxyl group (—COO) isbonded to the carbon atom at the 5-position of the oxazoline ring. Itcan be considered that the oxazoline compound acts as achain-lengthening agent by a reaction mechanism including such areaction. The chain-lengthening reaction with the oxazoline compound ismore efficiently performed by using a compound having at least 2oxazoline rings in its molecule. The reaction with such an oxazolinecompound is a chain-lengthening reaction, in which significant increasein the molecular weight of the ring-opening (co)polymer is observed,different from a mere chain-terminating reaction.

4. High-Molecular Weight Aliphatic Polyester

The chain of a ring-opening (co)polymer of a cyclic ester is lengthenedby the reaction of the ring-opening (co)polymer with the oxazolinecompound to provide a high-molecular weight aliphatic polyester. Themolecular weight of the high-molecular weight aliphatic polyester variesaccording to the molecular weight of the ring-opening (co)polymer used,the amount of the oxazoline compound used, reaction conditions, etc.,and no particular limitation is imposed thereon.

According to the process of the present invention, a high-molecularweight aliphatic polyester having a weight average molecular weight (Mw)of preferably at least 120,000, more preferably at least 130,000,particularly preferably at least 150,000 can be obtained. No particularlimitation is imposed on the weight average molecular weight (Mw).However, it is of the order of generally 1,000,000, often 500,000.

When glycolide is subjected to ring-opening polymerization, aring-opening polymer having a weight average molecular weight (Mw) of upto about 100,000 or about 110,000 is obtained. Such a ring-openingpolymer is reacted with a small amount of an oxazoline compound, wherebya high-molecular weight aliphatic polyester, whose weight averagemolecular weight has been increased to the extent of, for example,150,000 to 250,000, can be easily obtained. The molecular weight can befurther increased by controlling reaction conditions of thechain-lengthening reaction, such as the amount of the oxazoline compoundused.

A rate of increase in molecular weight by the chain-lengthening reactionof the ring-opening (co)polymer with the oxazoline compound can berepresented by a ratio (Mw₂/Mw₁) of a weight average molecular weight(Mw₂) of a ring-opening (co)polymer (i.e., high-molecular weightaliphatic polyester) after chain lengthening to a weight averagemolecular weight (Mw₁) of the ring-opening (co)polymer before the chainlengthening. According to the process of the present invention, themolecular weight of the ring-opening (co)polymer can be increased untilthe rate of increase in molecular weight amounts to preferably at least1.10, more preferably at least 1.20, particularly preferably at least1.35. No particular limitation is imposed on the upper limit of thisrate (Mw₂/Mw₁) of increase in molecular weight. However, it is generally10.00, preferably 5.00, more preferably 3.50.

According to the process of the present invention, a high-molecularweight aliphatic polyester having a molecular weight distributionrelatively broad compared with the ring-opening (co)polymer before thechain lengthening can be obtained. The molecular weight distribution(Mw/Mn) represented by a ratio of a weight average molecular weight (Mw)of a ring-opening (co)polymer (i.e., high-molecular weight aliphaticpolyester), whose molecular weight has been increased by thechain-lengthening reaction, to a number average molecular weight (Mn)thereof is preferably at least 1.90, more preferably at least 2.00,particularly preferably at least 2.10. No particular limitation isimposed on the upper limit of this molecular weight distribution(Mw/Mn). However, it is of the order of generally 5.50, often 4.50. Ifthe molecular weight distribution becomes too broad, the properties ofsuch a polymer as a whole may possibly be impaired.

The high-molecular weight aliphatic polyester obtained by the processaccording to the present invention is markedly improved in heatresistance compared with the ring-opening (co)polymer before thereaction with the oxazoline compound. A 1%-weight loss-startingtemperature on heating of a polymer can be used as an index to the heatresistance. Assuming that a 1%-weight loss-starting temperature onheating of a ring-opening (co)polymer before chain lengthening is T₁,and a 1%-weight loss-starting temperature on heating of a high-molecularweight aliphatic polyester obtained by the chain-lengthening reaction ofthe ring-opening (co)polymer with an oxazoline compound is T₂, (T₂-T₁)can be controlled to preferably at least 3° C., more preferably at least5° C. The resulting high-molecular weight aliphatic polyester shows atendency to increase its heat resistance as the increase in themolecular weight is promoted by the reaction with the oxazolinecompound. For example, (T₂-T₁) can be controlled to at least 15° C.,further at least 20° C. However, the effect to improve the heatresistance shows a tendency to be somewhat saturated with the increasein the weight average molecular weight (Mw) by the chain-lengtheningreaction, and the upper limit of (T₂-T₁) is of the order of generally30° C., often 25° C.

The high-molecular weight aliphatic polyester according to the presentinvention may contain additives such as inorganic fillers, lubricants,plasticizers, colorants (dyes and pigments), heat stabilizers andconductive fillers; other thermoplastic resins; and/or the like ifdesired These additive components may be added before addition of theoxazoline compound, upon the addition or after the addition so far asthey impair the chain-lengthening reaction of the ring-opening(co)polymer with the oxazoline compound. These additive components mayalso be added to the high-molecular weight aliphatic polyester formedafter the chain-lengthening reaction of the ring-opening (co)polymerwith the oxazoline compound.

EXAMPLES

The present invention will hereinafter be described more specifically bythe following Synthesis Examples, Examples and Comparative Examples.Measuring methods of physical properties are as follows:

(1) Weight Average Molecular Weight and Molecular Weight Distribution:

The weight average molecular weight (Mw) and molecular weightdistribution (Mw/Mn) of each sample were measured under the followingconditions making use of a gel permeation chromatography (GPC) analyzer.Sodium trifluoroacetate (product of Kanto Chemical Co., Inc.) is addedand dissolved in hexafluoroisopropanol (a product of Central Glass Co.,Ltd. was distilled for use) to prepare a 5 mM sodium trifluoroacetatesolvent (A).

The solvent (A) is passed through a column (HFIP-LG+HFIP-806M×2; productof SHODEX) at 40° C. and a flow rate of 1 ml/min. Each 10 mg of 5polymethyl methacrylate reagents (products of POLYMER LABORATORIES Ltd.respectively having already known molecular weights of 827,000, 101,000,34,000, 10,000 and 2,000 and the solvent (A) are used to prepare 10 mlof a solution. A 100-μl portion of the solution is passed through thecolumn to determine a detection peak time by detection of refractiveindex (RI). The detection peak time and molecular weight of each of the5 standard samples are plotted, thereby preparing a calibration curvefor molecular weight.

The solvent (A) is added to 10 mg of the sample to prepare 10 ml of asolution, and a 100-μl portion of this solution is passed through thecolumn to determine a weight average molecular weight (Mw), a numberaverage molecular weight (Mn) and a molecular weight distribution(Mw/Mn) from an elution curve thereof. C-R4AGPC Program Ver 1.2manufactured by Shimadzu Corporation was used for calculation.

(2) 1%-Weight Loss-Starting Temperature on Heating:

A thermogravimetric analyzer TG50 manufactured by METTLER Co. was used,and nitrogen was caused to flow at a flow rate of 10 ml/min to heat analiphatic polyester sample at a heating rate of 2° C./min from 50° C.under this nitrogen atmosphere, thereby determining a rate of weightloss. A temperature at which the weight of the aliphatic polyester hasbeen reduced by 1% based on its weight (W₅₀) at 50° C. is precisely readout, and that temperature is regarded as a 1%-weight loss-startingtemperature on heating.

(3) Torque Upon Melt Kneading:

A ring-opening (co)polymer and an oxazoline compound were melt-kneadedby means of a Labo Plastomill manufactured by Toyo Seiki Seisakusho,Ltd. to measure maximum torque at this time.

Synthesis Example 1

A 10-liter autoclave was charged with 5 kg of glycolic acid (product ofWako Pure Chemical Industries, Ltd.), and the contents were heated toraise their temperature to from 170° C. to 200° C. over about 2 hourswith stirring, whereby glycolic acid was condensed while distilling offwater formed. The pressures of the system was then reduced to 20 kPa(200 mbar), and the reaction mixture was held for 2 hours to distill offlow-boiling matter, thereby preparing a glycolic acid oligomer. Themelting point Tm of the thus-obtained glycolic acid oligomer was 205° C.

A 10-liter flask was charged with 1.2 kg of the glycolic acid oligomer,and 5 kg of benzylbutyl phthalate (product of Junsei Chemical Co., Ltd.)as a solvent and 150 g of polypropylene glycol (#400, product of JunseiChemical Co., Ltd.) as a solubilizing agent were added. The mixture washeated to about 270° C. under reduced pressure of 5 kPa (50 mbar) in anitrogen gas atmosphere to conduct “solution-phase depolymerization” ofthe glycolic acid oligomer. Glycolide formed was azeotropicallydistilled out together with benzylbutyl phthalate. Cyclohexane in avolume about twice as much as the azeotropic mixture thus obtained wasadded to the mixture, whereby the glycolide was separated out ofbenzylbutyl phthalate and collected by filtration. This product wasrecrystallized with ethyl acetate and dried under reduced pressure toobtain purified glycolide.

Synthesis Example 2

A glass-made test tube was charged with 100 g of the glycolide obtainedin Synthesis Example 1 and 5 mg of tin tetrachloride to conductpolymerization at 200° C. for 3 hours. After the polymerization,protracted polymerization was conducted at 160° C. for 12 hours. Afterthe polymerization, the reaction mixture was cooled, and a polymerformed was then taken out, ground and washed with acetone. Thereafter,the polymer was vacuum-dried at 30° C. to obtain the polymer. Theabove-described process was repeated to prepare a necessary amount ofpolyglycolic acid (polyglycolide).

Example 1

Into a Labo Plastomill manufactured by Toyo Seiki Seisakusho, Ltd., wereadded 40 g of the polyglycolic acid obtained in Synthesis Example 2, and0.28 g of 2,2′-m-phenylene-bis(2-oxazoline) (product of Kanto ChemicalCo., Inc.) were then added to melt-knead the resultant mixture at 240°C. for 20 minutes. After completion of the kneading, a melt, which was areaction product, was taken out to measure its physical properties. Theresults are shown in Table 1.

Example 2

The process was conducted in the same manner as in Example 1 except thatthe amount of 2,2′-m-phenylene-bis(2-oxazoline) added was changed from0.28 g to 0.40 g. The results are shown in Table 1.

Example 3

The process was conducted in the same manner as in Example 1 except thatthe amount of 2,2′-m-phenylene-bis(2-oxazoline) added was changed from0.28 g to 1.20 g. The results are shown in Table 1.

Comparative Example 1

The process was conducted in the same manner as in Example 1 except thatthe polyglycolic acid obtained in Synthesis Example 2 was used byitself. The results are shown in Table 1. TABLE 12,2′-m-Phenylene-bis(2-oxazoline) (parts by weight) 0 0.7 1 3 Torqueupon melt kneading 1.1 3.5 3.6 17 (N · m) Weight average molecular110,000 173,000 181,000 235,000 weight (Mw) Rate of increase in 1.001.57 1.65 2.14 molecular weight Molecular weight 1.75 2.20 2.30 3.47distribution (Mw/Mn) 1%-Weight loss-starting 230 237 252 252 temperatureon heating (° C.) (0) (+7) (+22) (+22) (temperature increased) Comp. Ex.1 Ex. 2 Ex. 3 Ex. 1

INDUSTRIAL APPLICABILITY

According to the present invention, there can be provided high-molecularweight aliphatic polyesters that are ring-opening (co)polymers of cyclicesters such as glycolide and lactide, whose molecular weights have beenhighly increased, and whose heat resistance and forming or molding andprocessing ability have been improved. According to the presentinvention, there can also be provided a process for producing ahigh-molecular weight aliphatic polyester, by which the molecular weightof the resulting polymer can be easily increased to a desired molecularweight without need of always using high-purity glycolide or lactide asa starting material, and heat resistance and forming or molding andprocessing ability are also improved.

Since the high-molecular weight aliphatic polyesters according to thepresent invention are high in molecular weight and excellent in heatresistance and have a moderately broad molecular weight distribution,they can be used in a wide variety of fields as extruded products suchas sheets, films and fibers, compression-molded products,injection-molded products, blow-molded products, composite materials(multi-layer films and multi-layer containers), and other formed ormolded products.

1. A high-molecular weight aliphatic polyester, whose molecular weighthas been highly increased by a chain-lengthening reaction of aring-opening (co)polymer of at least one cyclic ester selected from thegroup consisting of glycolide and lactide with an oxazoline compound tothe extent that a rate of increase in molecular weight represented by aratio (Mw₂/Mw₁) of a weight average molecular weight (Mw₂) of aring-opening (co)polymer after the chain lengthening to a weight averagemolecular weight (Mw₁) of the ring-opening (co)polymer before the chainlengthening amounts to at least 1.10.
 2. The high-molecular weightaliphatic polyester according to claim 1, wherein the molecular weightis highly increased to the extent that the rate of increase in molecularweight amounts to at least 1.20.
 3. The high-molecular weight aliphaticpolyester according to claim 1, wherein the molecular weight is highlyincreased to the extent that the rate of increase in molecular weightamounts to at least 1.35.
 4. The high-molecular weight aliphaticpolyester according to claim 1, wherein the weight average molecularweight (Mw) of the ring-opening (copolymer, whose molecular weight hasbeen increased by the chain-lengthening reaction, is at least 120,000.5. The high-molecular weight aliphatic polyester according to claim 1,wherein the ring-opening (co)polymer having a weight average molecularweight of at most 110,000 before the chain lengthening is subjected tothe chain-lengthening reaction into a high-molecular weight ring-opening(co)polymer having a weight average molecular weight of at least150,000.
 6. The high-molecular weight aliphatic polyester according toclaim 1, wherein a difference (T₂-T₁) between a 1%-weight loss-startingtemperature T₂ on heating of the ring-opening (co)polymer after thechain lengthening and a 1%-weight loss-starting temperature T₁ onheating of the ring-opening (co)polymer before the chain lengthening isat least 3° C.
 7. The high-molecular weight aliphatic polyesteraccording to claim 6, wherein the 1%-weight loss-starting temperature T₂on heating of the ring-opening (co)polymer after the chain lengtheningis at least 233° C.
 8. The high-molecular weight aliphatic polyesteraccording to claim 1, wherein a molecular weight distribution (Mw/Mn)represented by a ratio of a weight average molecular weight (Mw) of thering-opening (co)polymer, whose molecular weight has been highlyincreased by the chain-lengthening reaction, to a number averagemolecular weight (Mn) thereof is at least 1.90.
 9. The high-molecularweight aliphatic polyester according to claim 1, wherein the oxazolinecompound is an oxazoline compound having at least two oxazoline ringstructures in its molecule.
 10. The high-molecular weight aliphaticpolyester according to claim 9, wherein the oxazoline compound having atleast two oxazoline ring structures in its molecule is2,2′-m-phenylene-bis(2-oxazoline).
 11. A process for producing ahigh-molecular weight aliphatic polyester, which comprises subjecting aring-opening (co)polymer of at least one cyclic ester selected from thegroup consisting of glycolide and lactide to a chain-lengtheningreaction with an oxazoline compound to highly increase the molecularweight thereof to the extent that a rate of increase in molecular weightrepresented by a ratio (Mw₂/Mw₁) of a weight average molecular weight(Mw₂) of a ring-opening (co)polymer after the chain lengthening to aweight average molecular weight (Mw₁) of the ring-opening (co)polymerbefore the chain lengthening amounts to at least 1.10.
 12. Theproduction process according to claim 11, wherein the molecular weightis highly increased to the extent that the rate of increase in molecularweight amounts to at least 1.20.
 13. The production process according toclaim 11, wherein the molecular weight is highly increased to the extentthat the rate of increase in molecular weight amounts to at least 1.35.14. The production process according to claim 11, wherein thering-opening (co)polymer and the oxazoline compound are subjected to thechain-lengthening reaction at a temperature within a range of 100 to300° C.
 15. The production process according to claim 11, wherein thering-opening (co)polymer and the oxazoline compound are subjected to thechain-lengthening reaction under conditions that the reactiontemperature is not lower than the melting temperature of thering-opening (co)polymer, but not higher than 300° C., and the reactiontime is 5 to 40 minutes.
 16. The production process according to claim11, wherein the oxazoline compound is an oxazoline compound having atleast two oxazoline ring structures in its molecule.
 17. The productionprocess according to claim 11, wherein the chain-lengthening reaction isconducted in the presence of the oxazoline compound in a proportionwithin a range of 0.005 to 10 parts by weight per 100 parts by weight ofthe ring-opening (co)polymer.
 18. The production process according toclaim 11, wherein the ring-opening (co)polymer having a weight averagemolecular weight of at most 110,000 before the chain lengthening issubjected to the chain-lengthening reaction into a high-molecular weightring-opening (co)polymer having a weight average molecular weight of atleast 150,000.
 19. The production process according to claim 11, whereina difference (T₂-T₁) between a 1%-weight loss-starting temperature T₂ onheating of the ring-opening (co)polymer after the chain lengthening anda 1%-weight loss-starting temperature T₁ on heating of the ring-opening(co)polymer before the chain lengthening is made at least 3° C. by thechain-lengthening reaction.
 20. The production process according toclaim 11, wherein a molecular weight distribution (Mw/Mn) represented bya ratio of a weight average molecular weight (Mw) of the ring-opening(co)polymer, whose molecular weight has been highly increased by thechain-lengthening reaction, to a number average molecular weight (Mn)thereof is at least 1.90.